How do I implement members of internal interfaces

Question

I have been refactoring the codebase of the project that I am currently on so that classes/interfaces which are not useful beyond the confines of the assembly should be declared as internal (rather than public). But I've run into a problem with the following code:

internal interface IFirstInterface
{
    ...
}

internal interface ISecondInterface
{
    IFirstInterface First{ get; }
    ...
}

public class Implementer : ISecondInterface
{
    public IFirstInterface First {get; private set;}
    ...
}

My questions:

1. Why do members of internal interfaces have to be publicly implemented? If you implement the interface on an internal class, shouldn't the implemented members be internal? This is not a big issue since the interface members won't be publicly accessible anyway, given the class is internal. It just seems counter intuitive.

2. The main problem is with the scenario above since I cannot have a public getter for IFirstInterface since it is purportedly an internal interface i.e. I get the following error from the compiler:

Inconsistent accessibility: property type 'IFirstInterface' is less accessible than property 'Implementer.First'

Is there any way around this?

Note: I realise that there is probably little value in this refactoring exercise but I thought it would be a good way for me to understand more deeply the implications of the internal modifier.

Solution 1

Just to note - the code you've actually provided does compile, because Implementer is an internal class. The problem comes when Implementer is public.

The way round this is to use explicit interface implementation:

public class Implementer : ISecondInferface
{
    private IFirstInterface first;
    IFirstInterface ISecondInterface.First { get { return first; } }
}

You can't have the setter in there, because you're explicitly implementing the interface which doesn't define the setter. You could do this as an alternative:

public class Implementer : ISecondInterface
{
    internal IFirstInterface First { get; private set; }
    IFirstInterface ISecondInterface.First { get { return First; } }
}

It's unfortunate that internal interfaces have public members - it does complicate things like this. It would be strange for a public interface to have an internal member (what would it be internal to - the implementer or the declarer?) but for internal interfaces it makes a lot more sense.

Solution 2

Why do members of internal interfaces have to be publicly implemented?

When you define an interface, you do not define access level for the members, since all interface members are public. Even if the interface as such is internal, the members are still considered public. When you make an implicit implementation of such a member the signature must match, so it needs to be public.

Regarding exposing the getter, I would suggest making an explicit implementation of the interface instead, and creating an internal property to expose the value:

internal IFirstInterface First { get; private set; }

IFirstInterface ISecondInterface.First
{
    get { return this.First; }
}

Solution 3

I know this post is a few years old but i think it’s worth noting that you can implement an internal interface on a public class, see the following links:

http://forums.create.msdn.com/forums/p/29808/167820.aspx
http://msdn.microsoft.com/en-us/library/aa664591%28VS.71%29.aspx

An example from the first link:

internal interface ISecretInterface
{
    string Property1 { get; }
}

public class PublicClass : ISecretInterface
{
    // class property
    public string Property1
    {
        get { return "Foo"; }
    }

    // interface property
    string ISecretInterface.Property1
    {
        get { return "Secret"; }
    }
}

Comments

[chtenb]

If its an option, you can get around the limitation of interface methods being public by using an abstract base class instead.

[jpoh]

Ah, correct. I'll edit my question so it's less confusing to readers. Thanks!

[Robot Mess]

How is your answer from 2011 different from the two answers from 2009?

[Robert]

Honestly i don't know and i can see why i would have added it although i do remember doing it. It doesn't add anything and i've been trying to figure it out by looking at the revision history to see if there was a clue but still nothing. Maybe i was having a dumb day.

[binki]

For some reason this answer is easier for me to consume than the earlier ones because the explanation at the beginning of the answer addresses my situation and matches what I Googled. So thanks for clearly stating that public classes can implement internal interfaces in an easily-consumable way ;-).